A principal objective of coal beneficiation is to increase the calorific heating value or amount of thermal energy of the coal which may be released during a subsequent combustion process.
One method of increasing the thermal energy released during combustion of coal is to decrease the amount of moisture by subjecting the coal to a drying process. It will be appreciated that moisture in coal has no heating value and, although not environmentally harmful, facilitates depletion by evaporation of a portion of the thermal energy released during combustion of coal.
Another known method of increasing the thermal energy released during combustion of coal is to decrease the amount of volatile matter within the coal. The amount of volatile matter within coal may be decreased by subjecting the coal to mild gasification by pyrolysis. Pyrolysis of coal in an oxygen deficient atmosphere removes volatile matter, e.g. low boiling point organic compounds and heavier organic compounds, by breaking chemical bonds during the heating process. Breaking of chemical bonds within coal during the heating process increases the relative percentage of elemental carbon which provides most of the calorific heating value when coal is burned.
Although various methods of increasing the thermal energy released during combustion of coal are known, many of the known methods require large volumes of continuously flowing streams of oxygen deficient gas for convective heat transfer. An oxygen deficient gas as used herein refers to a gas having generally less than 5% oxygen by weight.
Heretofore, oxygen deficient gas streams have typically been produced using well known air separation technologies such as cryogenic distillation, membrane separation and pressure swing absorption. Although the known methods of producing oxygen deficient gas streams for coal drying and mild gasification processes have been proven to perform satisfactorily in certain applications, these technologies are cost ineffective when considered for large processing needs like mild coal gasification, coke preheating and the like. Large mild coal gasification systems may range up to 8,000 square feet and may use 5,000 to 10,000 standard cubic feet of oxygen deficient gas per hour per square foot of cross section for thermal treatment of coal and or oil shale whether the coal and or oil shale is to be dried or fractioned into solid and gaseous phase components.
The present invention is directed to a process for treating noncaking, noncoking coal to form char. The process employs high sensible heat oxygen deficient gas streams for consecutive drying and pyrolytic coal treatment processes. The high sensible heat containing gas streams are renewed using products of combustion from the combustion of a process derived gaseous fuel having a variably controllable calorific heating value. The calorific heating value of the process derived gaseous fuel may be controlled by selectively varying the condensation and removal of hydrocarbon components from the process derived gaseous fuel as required in accordance with the present invention.
It will be appreciated that the mild gasification process, albeit a thermal process, for the formation of char starts with a noncaking, noncoking coal and differs substantially from a process utilized for the formation of coke. The essential difference and the subject of this invention is the operability of a mild gasification process as manifested in the ability to control both the mass flow ratio as compared with the dried coal and the temperature of the high sensible heat pyrolysis gas so as to have precise control over the residual volatile content of the char formed. It will be further appreciated that coupled with the control of the mass ratio and temperature of the pyrolysis gas stream is the need for precision control over the environmental and safety aspects of the mild gasification process. Moreover, last but not least, is the need to combust and consume all of the low calorific heating value process derived gaseous fuel so as to render the process economical to operate.
The residual volatile content of the product char must be controlled precisely so as to render the char useful for subsequent combustion in power generation, as an industrial fuel, or for a metallurgical application. Typical product char and coke analyses are presented below in Table 1. Although the difference in volatile content between char and coke is readily apparent, what is not apparent is the degree of process complexity involved in performing a partial separation of volatiles to form char when compared to a full separation of volatiles to form coke where precision control of temperature, chemical composition and mass of the gas streams is not readily defined.
TABLE 1 ______________________________________ Coke.sup.1 Coke.sup.1 Char.sup.2 Char.sup.2 ______________________________________ Moisture 0.7 0.8 1.53 5.00 Volatile 2.0 1.4 26.55 18.67 % Fixed 85.3 87.1 66.90 69.26 Carbon % Ash % 12.0 10.7 5.02 7.06 Sulfur % 0.7 1.1 0.20 0.20 Heating 12,550 12,690 12,983 12,162 Value, Btu/lb ______________________________________ .sup.1 Coke processed at 1832.degree. F. to 2102.degree. F. .sup.2 Char processed at 885.degree. F. to 985.degree. F.
The process includes a coal drying process and a mild coal gasification process employing, in combination, means for formation of oxygen deficient gas streams and renewal of both the sensible energy and the oxygen deficient quality of the gas streams. The present invention requires precise control of gas chemistry, gas supply temperature and advanced flammable gas and vapor handling technology.
As used herein, formation of the oxygen deficient gas streams means that the oxygen deficient gas streams, i.e. nitrogen from air, carbon dioxide from combustion and water vapor from combustion or from the drying process are formed within the system except for controlled venting. Similarly, as used herein, renewal of both the sensible energy and oxygen deficient quality of the gas streams means that gaseous fuel formed in the mild gasification process is mixed with air in a combustion chamber to release heat and to oxidize the combustible vapors and gases which result from the mild gasification process.
In accordance with the present invention, the renewal of the oxygen deficient quality of the gas streams and utilization of process derived low calorific heating value gaseous fuel facilitates the possibility of an economically feasible advanced heat treatment process for combustible materials such as gas permeable, solid carbonaceous materials.
Accordingly, one aspect of the present invention is to provide a gas combustion and renewal process for supplying large volumes of continuous flowing oxygen deficient gas streams for convection drying and mild gasification of nocaking, noncoking coal. The process combines the elements of gas recirculation, combustion of flammable vapors and gases produced in the process and certain combustion chamber gas bypass and mixing technology conducive to modulation of the gas supply temperature and gas composition. The present invention is particularly applicable for production of large continuously flowing volumes of oxygen deficient gas streams as may be required for bulk drying and mild gasification of substantially continuously flowing streams of noncaking, noncoking coal. The oxygen deficient gas streams are renewed meaning that the oxygen deficient compositions, i.e. nitrogen from air, carbon dioxide from the combustion reaction and water vapor from either the combustion reaction or from drying are utilized repetitively except for venting.